You can just follow many tutorials on YouTube and Google; it's not that complicated if you actually search.
Most people make towers, where a platform is high up in the air and there's a hole in the middle of the platform. Put some water on the sides, and mobs will spawn being pushed down the pitfall of doom. Make a room at the bottom to collect your XP and free items.
I made a tower and a underground spawner with the mobivator and both feed into a double sizzler design. I found the captainsparkelz design still let spiders through and so did ethos single sizzler. So I made a double sizzler and the roaches check in but they dont check out.
Most people make towers, where a platform is high up in the air and there's a hole in the middle of the platform. Put some water on the sides, and mobs will spawn being pushed down the pitfall of doom. Make a room at the bottom to collect your XP and free items.
SCHZAM!